This invention relates to the valve art, and, more particularly, to fire-resistant or fire-safe ball valves.
When employed in the valve art, the term "fire-safe" has come to mean a valve that satisfies certain specified conditions when subjected to a fire. See Arant, Fire-Safe Valves--An Overview, Proceedings, Thirty-Sixth Annual Symposium on Instrumentation for the Process Industries (Texas A&M University, 1981). Unfortunately, different sets of conditions have been promulgated by different organizations and the valve industry has not yet recognized a uniform standard. Basically, according to one of the standards (American Petroleum Institute 607), if a valve will substantially maintain a fluid seal in a closed position at a valve body temperature of at least 1100 degrees F. for at least ten minutes, it may be certified as "fire-safe". Valves which include design characteristics to resist leakage upon exposure to fire but cannot meet this standard are commonly referred to as "fire-resistant".
The invention is particularly applicable to a new and improved soft-seated fire-safe ball valve and seat assembly for a valve of the type having a so-called "floating ball" and will be described with particular reference thereto. However, it will become readily apparent to those skilled in the art that the invention is capable of broader applications and could be adapted for use in other types and styles of valves.
Ball valve constructions in commercial use typically employ annular seats or seat rings formed of a resilient and deformable plastic such as Teflon (a registered trademark of E. I. duPont de Nemours and company) for sealing engagement with the ball. A pair of such seat rings are positioned adjacent the valve inlet and outlet openings. The ball itself is mounted for a slight amount of free movement or shifting axially of the seats when the ball is in a valve closed position under fluid pressure conditions. Such shifting causes the ball to act against and to flex and deform the downstream seat ring to enhance its sealing engagement with the ball. The amount of such flexing varies in accordance with the fluid pressure involved.
When subjected to a fire, the soft annular seat of a conventional floating ball type of ball valve is substantially damaged by the heat of the fire to the extent that leakage through the valve may become unacceptable. Typically, downstream of the ball, the sequence of seat destruction is such that the plastic first softens and begins to flow out or extrude through the valve port. Continued exposure to excessive heat ultimately causes the seat to char and sublimate or evaporate. The destruction of the plastic seat allows the ball to further shift under fluid pressure conditions until the ball engages a secondary seat. Such a secondary seat typically comprises a metal or non-flammable radially inwardly extending projection of the valve body, such as a support shoulder for the plastic seat. Normally, such a surface is not specially designed for a high degree sealing engagement against the ball and allows substantial leakage.
Another particular problem occurs where the plastic seat is only partially destroyd by a fire. For example, where a valve is exposed to radiant heat from a fire on one side only, or in a low intensity fire, only that portion of the valve seat nearest the fire may soften and extrude into the valve port. The ball may then shift under fluid pressure toward that area made available to it by the extrusion and, being unable to evenly contact the secondary seat, expose a large leak path. Alternatively, the ball may be held back from making any contact with the secondary seat by the undestroyed portions of the plastic seat and similarly expose large leak paths. Under either situation fluid may rush through the leak paths and quench the valve. The quenching action operates to prevent further deterioration of the seat in spite of a continuing fire and typically maintains a massive leak through the valve.
An additional but often unrecognized problem which occurs during a fire is the rapid increase in fluid pressure by heated fluid which is trapped between the inlet and outlet seats around the ball. The heat of the fire may heat and even vaporize such fluid in the center of the valve between the seats. Often the fire is so intense that the fluid is so rapidly vaporized that it cannot escape past the seats quickly enough to prevent an excessive increase of pressure within the valve. Such an increase in pressure can easily exceed the valve rating and rupture the seals at the stem packing and the body joints, or rupture the valve body itself.
Another practical problem occurs when a fire hose is trained upon a valve in a volatile liquid system that has been heated by a fire. The quick-cooling action of the hose water causes a violent condensation of heated vapor in the valve that dislodges the ball and churns up char, waste and contaminants that may become lodged between the ball and the ball sealing surface and thereby provide additional leak paths.
One overall objective of fire-resistant or fire-safe ball valve seat designs is to obtain a valve which will seal with conventional valve seat materials at normal operating conditions and will also seal when subjected to a fire. Various forms and types of ball valve seat designs have heretofore been suggested and employed in the industry for purposes of obtaining a fire-safe or fire-resistant ball valve, all with varying degrees of success. It has been found that the defects present in most prior fire-safe or fire-resistant ball valve designs are such that the devices themselves are of limited economic and practical value.
A common type of fire-resistant ball valve design includes a primary soft seat of a plastic material such as Teflon, and a secondary seat of metal or a high temperature composite material to seal the valve upon destruction of the primary seat in a fire. The secondary seat typically comprises a metal rim or washer interposed between the soft plastic primary seat and a valve body support shoulder. This design suffers from the problems naturally resulting from any type of metal-to-metal seal. Since the ball in a floating ball type valve is never perfectly spherical and the secondary metal seat is not made perfectly circular, leakage across the secondary seal after destruction by fire of the primary seal is usually high, as the ball cannot make a full annular contact with the metal seat. For a metal-to-metal seal to be anywhere near leak-tight in a fire-safe or other application, the sealing surfaces must be match lapped or burnished, one into the other. Since such a procedure is very expensive, match lapping is an economically impractical procedure for a manufacturer of fire-safe or fire-resistant ball valves. In addition, a match-fitted secondary seat would likely be marred by the hazards and consequences of normal valve operation, such as corrosion, pitting, scaling, erosion and the like, to the extent that during a subsequent fire the advantages of match fitting would have been lost. Ball valve designs incorporating the secondary metal seat or high temperature composite seat also suffer from the problems of partial deterioration and quenching of the primary seat and the problems of loose and blocking char and waste materials associated with quick cooling.
A suggested improvement over the mere metal secondary seat design has been to add a secondary seat comprised of a heat resistant material which is more deformable and resiliant than metal. Typically, carbon or graphite rings have been used. While such designs may provide improved operation when new, it has been found that such designs are particularly susceptible to damage in normal service. Normal wear through cycling the valve, erosion while opening, or abrasion by foreign matter can easily damage the secondary seat materials since they are typically brittle and of low strength compared to a normal plastic seat. Therefore, such designs usually still include a metal lip or rim as a final or tertiary seat to restrict leakage if the secondary seat is damaged. Such a multiplicity of seats increases the size, complexity, and cost of the valve without adding a reliable redundant seal. Whatever elements of wear, erosion or other foreign matter might damage one of the seats is likely to damage all of the seats since they are all equally exposed during normal service.
One alternate suggestion for obtaining a fire-safe valve is to include packing a conventional valve in enough insulation to insulate the valve for a sufficient amount of time to obtain a fire-safe rating. Another suggestion is to dispose a sprinkler near the valve which will quench the valve during a fire. Both of these designs are unsuitable for practical cost reasons in that they would involve expensive installations and maintenance. In addition, an insulated valve would suffer from the problem of uncertainty as to whether the insulation would be properly reattached or installed each time the valve received maintenance.
It has, therefore, been desired to develop a fire-safe ball valve and seat assembly which would satisfactorily operate at normal operating conditions and also seal the valve in a valve closed position upon exposure to a fire. Preferably, such a design would eliminate the necessity for utilizing costly sprinklers or insulation packings to protect the valve.
The present invention contemplates a new and improved construction which overcomes all of the above referred to problems and others and provides a new and improved fire-safe floating ball type valve and soft seat assembly which provides improved sealing capabilities and effectively resists leakage upon exposure to any realistic fire. The invention further contemplates being useful with a wide variety of seat designs and materials which effectively seal at a wide variety of normal operating conditions.